RemoteFrame ignores visibility updates after detach

The element visibility observer posts visibility updates to
RemoteFrameView. Based on crash reports, it looks like these can
sometimes arrive after the RemoteFrame has been removed from the
frame tree, making the FrameClient inaccessible. This CL causes an
early return in that case, avoiding a null pointer dereference.

Bug: 789460
Change-Id: Ibe82253859a9c0f90917331532dac39ba90fd364
Reviewed-on: https://chromium-review.googlesource.com/806363
Reviewed-by: dsinclair <dsinclair@chromium.org>
Commit-Queue: Ken Buchanan <kenrb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521694}

third_party/WebKit/Source/core/frame/RemoteFrameView.cpp

@@ -193,6 +193,9 @@ void RemoteFrameView::SetupRenderThrottling() {
 void RemoteFrameView::UpdateRenderThrottlingStatus(bool hidden,
                                                    bool subtree_throttled) {
   TRACE_EVENT0("blink", "RemoteFrameView::UpdateRenderThrottlingStatus");
+  if (!remote_frame_->Client())
+    return;
+
   bool was_throttled = CanThrottleRendering();
 
   // Note that we disallow throttling of 0x0 and display:none frames because